Building Sustainable Futures: Evaluating Embodied Carbon Emissions and Biogenic Carbon Storage in a Cross-Laminated Timber Wall and Floor (Honeycomb) Mass Timber Building

Abstract

The building sector significantly contributes to global energy consumption and carbon emissions, primarily due to the extensive use of carbon-intensive materials such as concrete and steel. Mass timber construction, particularly using cross-laminated timber (CLT), offers a promising low-carbon alternative. This study aims to calculate the embodied carbon emissions and biogenic carbon storage of a CLT-based affordable housing project, 340+ Dixwell in New Haven, Connecticut. This project was designed using a honeycomb structural system, where mass timber floors and roofs are supported by mass timber-bearing walls. The authors are not aware of a prior study that has evaluated the life cycle impacts of honeycomb mass timber construction while considering Timber Use Intensity (TUI). Unlike traditional post-and-beam systems, the honeycomb design uses nearly twice the amount of timber, resulting in higher carbon sequestration. This makes the study significant from a sustainability perspective. This study follows International Standard Organization (ISO) standards 14044, 21930, and 21931 and reports the results for both lifecycle stages A1–A3 and A1–A5. The analysis covers key building components, including the substructure, superstructure, and enclosure, with timber, concrete, metals, glass, and insulation as the materials assessed. Material quantities were extracted using Autodesk Revit^®, and the life cycle assessment (LCA) was evaluated using One Click LCA (2015)^®. The A1 to A3 stage results of this honeycomb building revealed that, compared to conventional mass timber housing structures such as Adohi Hall and Heartwood, it demonstrates the lowest embodiedf carbon emissions and the highest biogenic carbon storage per square foot. This outcome is largely influenced by its higher Timber Use Intensity (TUI). Similarly, the A1-A5 findings indicate that the embodied carbon emissions of this honeycomb construction are 40% lower than the median value for other multi-family residential buildings, as assessed using the Carbon Leadership Forum (CLF) Embodied Carbon Emissions Benchmark Study of various buildings. Moreover, the biogenic carbon storage per square foot of this building is 60% higher than the average biogenic carbon storage of reference mass timber construction types.

1. Introductions

The most recent report from the Intergovernmental Panel on Climate Change [1] issues a clear warning, stressing the critical need for immediate action from all countries and industries within the next decade to address global temperature rise and avert a climate crisis. The UN’s tenth Emissions Gap Report [2] also presents a concerning outlook, revealing that greenhouse gas (GHG) emissions have risen by 1.5% annually over the last decade. The building sector is crucial in tackling the climate crisis, responsible for 36% of global energy use and 39% of energy-related carbon emissions [3]. This is largely driven by the extensive reliance on fossil fuel-based, carbon-intensive materials like concrete and steel [3]. Bulk material production, particularly iron, steel, cement, lime, and plaster, is the largest contributor to these carbon emissions [2]. The manufacture of glass, cement, and steel alone accounts for 11% of energy- and process-related emissions [4].

A building’s carbon footprint consists of both embodied and operational carbon emissions. Embodied carbon emissions are generated during construction processes, including raw material extraction, manufacturing, transportation, and on-site activities. Operational carbon, in contrast, arises from energy use during the building’s occupied lifespan, such as heating, cooling, lighting, and maintenance [5].

Figure 1 demonstrates the anticipated shift in carbon emissions within the building sector, showing that the percentage of operational carbon of total emissions is expected to decline over time, while embodied carbon emissions will increase unless considerable reduction efforts are made. According to current estimates, in 2021, operational carbon accounted for 75% of total building sector emissions, while embodied carbon emissions contributed to 25% of emissions. By 2050, this balance is expected to change, with operational carbon decreasing to 51% and embodied carbon emissions rising to 49%, indicating the increasing impact of construction materials and processes [5]. Therefore, making substantial efforts to reduce embodied carbon emissions is crucial for total building decarbonization.

As urbanization continues to drive population migration to cities, with an estimated 60% urbanization rate projected by 2030 [6], global construction activities are experiencing a massive surge, necessitating innovative construction methods and designs to conserve natural resources [6,7].

Mass timber construction, driven by innovative engineered wood products like cross-laminated timber (CLT), offers significant potential for reducing embodied carbon emissions in the building sector [3,8]. Mass timber construction presents a sustainable alternative, potentially achieving a significantly lower carbon footprint compared to traditional building methods such as steel structures [8,9].

Mass timber (Figure 2) refers to a category of framing styles often using small wood members formed into large panelized solid wood construction, including CLT, Nail Laminated Timber (NLT), Dowel Laminated Timber (DLT), or glulam panels for floor, roof, and wall framing [10]. The most well-known example is CLT [11]. Mass timber is increasingly used in building applications [12] as people begin to recognize and take advantage of its benefits [8,13,14].

1.1. Three Building Typologies

The mass timber structural systems applied most often in projects can be grouped into three main categories (Figure 3):

Mass timber floors and roofs on post-and-beam framing (Figure 4 and Figure 5): These systems, like the framing style found in traditional heavy timber buildings, incorporate beams, posts, and decking, commonly linked with steel fasteners. The posts and beams are usually made of glulam. The decking can consist of CLT, NLT, DLT, or other types of timber frame decking [16].

It is the most common structural scheme for mass timber buildings in the US. In multi-family buildings, corridor walls and unit separation walls are typically non-load bearing and are framed as infill up to the underside of beams or panels. This setup offers greater flexibility for future renovations and interior reconfigurations compared to a bearing wall approach [17].

Mass timber floors and roofs supported by light-frame-bearing walls (either wood or steel) (Figure 6): This hybrid construction style combines light-frame-bearing walls, typically wood (though sometimes cold-formed steel), with mass timber floor and roof panels. This approach leverages the strength of both materials. Light-frame wood-bearing walls are a reliable, cost-effective choice for multi-family buildings of up to five stories, with the light-frame walls commonly used for shaft, corridor, exterior, and unit separation walls. They allow for the easy integration of electrical wiring and plumbing and are well-documented in terms of fire resistance, acoustics, building enclosure, and load-bearing capabilities [17].

Mass timber floors and roofs supported by mass timber-bearing walls—Honeycomb:

Unlike post-and-beam systems, CLT offers two-way spanning capabilities, allowing designers more flexibility. These panels can be arranged in a honeycomb structure to efficiently handle both vertical and lateral loads. In such designs, CLT panels—like steel and reinforced concrete—serve as both floors and walls, with the walls supporting the building’s structural load. In certain cases, CLT may even replace concrete as the core for the building’s shear wall [16].

Known as CLT-bearing walls, vertical and horizontal systems with CLT, or a honeycomb construction (Figure 7, Figure 8 and Figure 9), this approach was applied in some of the earliest and tallest mass timber multi-family buildings. A notable example by Waugh Thistleton Architects, in 2009, is Stadthaus in the United Kingdom, an eight-story CLT structure atop a single-level concrete podium. Lendlease, for the US Department of Defense, built a group of these honeycomb structures as hotels on US Army Bases in different parts of the country, two examples of which are shown in Figure 7. The first US multi-family example is 340+ Dixwell in New Haven, CT [17].

1.2. Life Cycle Assessment (LCA)

Life cycle assessments can be conducted to determine whether conventional construction materials perform better than mass timber construction in terms of environmental impacts. The LCA serves as a tool to evaluate potential environmental impacts throughout a building’s life cycle (Figure 10), from material extraction and production to construction (Stage A), the usage phase (Stage B), and ultimately waste treatment and end-of-life disposal (Stage C) [18]. Building life cycle assessments (BLCAs) are conducted following the International Organization for Standardization (ISO) guidelines, primarily following ISO 14044 [19], ISO 21930 [20], and ISO 21931 [21] standards.

ISO 14044 serves as a comprehensive standard for life cycle assessments (LCAs), outlining requirements and guidelines for evaluating and reporting environmental impacts, including greenhouse gas (GHG) emissions. While ISO 14044:2006 and its amendments (Amd.1:2017 and Amd.2:2020) [19] offer detailed guidance on conducting LCA, they do not specifically address the treatment of biogenic carbon in a dedicated section. Biogenic carbon refers to the carbon stored in biological materials like plants and soil. In building materials, biogenic carbon can be viewed as a “negative emission” since carbon is sequestered within the material during its growth phase [22]. On the other hand, ISO 21930:2017 [20] provides detailed guidance on biogenic carbon accounting, particularly in Section 7.2.7, “Accounting of biogenic carbon uptake and emissions during the life cycle”. This section outlines the treatment of bio-based materials, such as wood, derived from renewable resources within life cycle assessments. Key considerations include recording the amount of biogenic carbon entering the product system as a removal from the atmosphere, quantified as −1 kg CO₂ eq. per kg of biogenic carbon. Emissions resulting from the combustion or biodegradation of bio-based materials are accounted for as biogenic CO₂ emissions, noted as +1 kg CO₂ eq. per kg of biogenic carbon. If bio-based materials leave the system, the associated biogenic carbon is documented as an export. This method provides a thorough assessment of biogenic carbon throughout its entire life cycle, covering its absorption, storage, and eventual release. ISO 21931-1:2022 [21] does not specifically mention biogenic carbon. The standard broadly includes carbon in its overall assessment of greenhouse gas emissions without distinguishing between biogenic and non-biogenic sources under the “carbon performance” framework for climate change impacts.

Most studies on LCAs have compared the life cycle assessment (LCA) of conventional construction materials like reinforced concrete (RC) and steel with mass timber (MT) buildings, focusing primarily on global warming potential (GWP) and life cycle energy. One such example is a study by Robati and Oldfield [23], where they found that the embodied carbon emissions of a mass timber building typically range between 196 kg CO₂ eq./m² and 590 kg CO₂ eq./m², with an average of 417 kg CO₂ eq./m² across stages A-C, including the structure, substructure, façade and finishes building element. In contrast, for a post-tensioned concrete building, the range is from 307 kg CO₂ eq./m² to 618 kg CO₂ eq./m², with an average of 465 kg CO₂ eq./m². This represents a reduction of 48 kg CO₂ eq./m² or a 10% reduction in the average embodied carbon emissions for the mass timber building across these stages. This study [23] used 10,000 sample data points for each Monte Carlo simulation to find the results. These samples were randomly selected from a distribution function generated using mean and standard deviation values from multiple sources, including inventory databases, published studies, reports, and Environmental Product Declarations (EPDs). The whole building life cycle assessments (WBLCAs) in this study were not all modeled under the same assumptions, as the study intentionally incorporated variability and uncertainty to better reflect real-world conditions. The embodied carbon emissions results were influenced by several uncertain parameters, including variations in embodied carbon emissions intensity from different inventory databases, transportation distances for material sourcing, building component lifespans, carbon sequestration, and biogenic carbon losses from timber, concrete carbonation over time, and different end-of-life scenarios for materials. The findings of this study indicate that mass timber buildings generally have lower embodied carbon emissions than concrete structures, though the extent of this difference depends on the assumptions made and the input data used for the calculations.

Another example is the life cycle assessment (LCA) study analyzed by Duan and their team [8], where they examined mass timber across various scales, including building, component, structural, and urban levels, with an emphasis on cradle-to-gate and cradle-to-grave boundaries. According to ISO 21930:2017 [20], cradle-to-gate (Stage A) refers to assessing a product’s environmental impact from raw material extraction through manufacturing, ending at the factory gate, excluding use and disposal phases. In contrast, cradle-to-grave covers the full life cycle (Stage A to Stage C), from raw material extraction to manufacturing, use, and end-of-life disposal, providing a holistic and comprehensive view of environmental impacts. The findings of this study [8] show that reinforced concrete (RC) structures produce 42.7% more embodied greenhouse gas (GHG) emissions than mass timber.

Since global warming is the most commonly used metric for assessing a building’s environmental performance, other impact categories are rarely stated. However, some studies have explored additional impact categories [24]. For example, Chen and their team [25] found that mass timber buildings had 18% lower global warming potential, 1% lower ozone depletion, and 47% lower eutrophication impacts compared to equivalent concrete buildings. Allan and Phillips [26] reported that mass timber (MT) structures performed 31–41% better than steel stud (SS) structures in terms of global warming potential (GWP), eutrophication, and particulate matter related to human health. However, it was also observed that steel buildings performed better regarding the environmental impact categories of smog formation, acidification, and ozone depletion, showing a 48% to 58% lower impact compared to mass timber building designs.

Several studies have assessed the life cycle impacts of multi-family mass timber buildings by comparing them with concrete and steel alternatives. One such study was conducted by the University of Washington [27], which performed a full life cycle assessment (LCA) of Heartwood on a 67,000-square-foot, eight-story mass timber apartment building. The study evaluated the global warming potential (GWP) of the building’s superstructure from cradle to grave and compared it with a concrete equivalent. Initial results showed that Heartwood’s GWP was 38% lower than that of the concrete building. When factoring in the carbon sequestered in the timber, the net GWP reduction reached 108% at the construction stage and 103% at the end-of-life stage.

Another relevant study by Hemmati and their team [28] analyzed the embodied carbon emissions of Adohi Hall, a mass timber residential structure at the University of Arkansas. This study compared the building to an equivalent steel-framed design using a cradle-to-construction site system boundary (A1–A4) and the Tally^® LCA tool. The findings revealed that the mass timber building emitted 198 kg CO₂ eq./m² of gross floor area, compared to 243 kg CO₂ eq. for the steel version, representing a 19% reduction. Additionally, the timber structure stored around 2757 tons of CO₂ eq., highlighting its potential for long-term carbon storage.

Some LCAs have specifically focused on the comparison between cross-laminated timber (CLT) structures and conventional reinforced concrete buildings. A study by Pierobon and their team [29] examined the environmental benefits of using hybrid CLT structures in mid-rise non-residential buildings. The building lifecycle stages included ‘cradle-to-gate, from the acquisition of the raw materials until the construction of the building (stages A1 to A5). The findings revealed a 26.5% reduction in global warming potential for hybrid CLT buildings compared to traditional reinforced concrete buildings, excluding biogenic carbon emissions across these stages.

A life cycle assessment (LCA) study has also been carried out to identify the primary contributors to global warming in both conventional buildings (reinforced concrete and steel) and mass timber structures. The study by Gu and their team [3] identified concrete and rebar as major contributors to global warming potential (GWP) in both concrete and mass timber buildings, primarily due to their high carbon emissions from extraction and manufacturing. Concrete is a major contributor to global warming potential (GWP) because its CO₂ emissions are directly tied to the quantity of cement used in the mix. Cement production generates greenhouse gases in two primary ways: directly, through the release of carbon dioxide during the thermal decomposition of calcium carbonate into lime and carbon dioxide, and indirectly, through the energy-intensive process, which largely relies on the combustion of fossil fuels [30]. Approximately 900 kg of CO₂ is released for every ton of cement produced, contributing to 88% of the total emissions associated with an average concrete mix [31]. Similarly, steel significantly contributes to global warming potential (GWP) due to the high energy consumption and carbon emissions associated with its production and processing. A 2013 study by Tsai [32] reported that in 2004, the steel industry alone emitted approximately 590 million tons of CO₂, representing 5.2% of global anthropogenic greenhouse gas emissions. These emissions mainly result from the energy consumption of fossil fuels and the use of limestone to refine iron oxides. Mass timber buildings, on the other hand, generate lower emissions during production and manufacturing compared to concrete and steel while also contributing to biogenic carbon storage.

Numerous studies have performed life cycle assessments (LCAs) specifically on mass timber buildings to achieve a more comprehensive understanding. In the mass timber buildings analyzed in the study by Gu and their team [3], it was found that carbon storage exceeded the carbon emissions generated during manufacturing, including both fossil and biogenic carbon. The study assessed temporal carbon storage benefits using cumulative radiative forcing (RF) over the entire time horizon up to the final year of storage. The net GWP was calculated by subtracting the carbon storage benefit (in kg CO₂ eq/m²) from the total GWP estimated for life cycle stages A1–A5 in the WBLCA analysis. This analysis showed that the net GWP for mass timber buildings at year 80 (average lifespan) was net-negative, meaning that the 80-year carbon storage benefit offset the GWP from fossil emissions, effectively making mass timber buildings a carbon sink. Even though A1–A5 emissions occur at the beginning, their impact is assessed over time. Since mass timber stores carbon for decades, this long-term benefit is factored into the net GWP calculation, making the building a carbon sink over its expected 80-year lifespan.

Although mass timber construction compositions can share similarities, they often differ based on the materials used, which, in turn, affect the building’s embodied impact [24]. Additionally, Bahramian and Yetilmezsoy [33] reviewed LCA studies on buildings over the past two decades and found numerous varying parameters, such as lifespan, functional unit, life cycle stages, and impact categories. These differences make it challenging to compare buildings across different studies.

Objective of This Study

While many studies have evaluated the environmental impact of mass timber buildings versus traditional materials such as concrete and steel, this research specifically examines the embodied carbon emissions (Stages A1 to A3 and A1 to A5) associated with a prototype mass timber building (340+ Dixwell) in New Haven, Connecticut, USA, featuring a honeycomb CLT mass timber system. This study is significant because we are not aware of prior research that has evaluated the life cycle assessment (LCA) of honeycomb CLT mass timber. This approach is characterized by a higher Timber Use Intensity (TUI), meaning it uses a greater volume of wood per unit area compared to other construction methods. Selecting between honeycomb CLT and Post-and-Beam mass timber systems is an early design decision for mass timber projects. This decision has significant impacts on the TUI and, thus, on project costs, carbon storage, and resource efficiency. The TUI of this project is 1.22. TUI typically ranges from 0.59 to 0.75 CF/SF for post-and-beam mass timber buildings. For example, the Burwell Center, Nez Perce-Clearwater National Forests Office, Bullitt Center, and Return to Form have TUIs of 0.69, 0.59, 0.63, and 0.75 CF/SF for the above-podium floor area, respectively [34,35].

1.3. Project Description

340+ Dixwell Avenue (Figure 11) is in New Haven, CT, and features a honeycomb design (Figure 12). It includes two four-story buildings, Building 340 and Building 316, with a total net area of 7138.4 m² (76,838 SF) and a total gross area of 8064.4 m² (86,805 SF). The design team comprises Schadler Selnau Architects, Gray Organschi Architecture, Odeh Engineers, the Developer/Owner Team, Beulah Land Development Corporation, HELP USA, and Spiritos Properties. Spiritos Properties is grateful for the support received from the US Forest Service (USFS) through its Wood Innovation Grant (WIG) award in 2018.

The study on 340+ Dixwell’s embodied carbon emissions footprint employs embodied carbon emissions calculations using One Click LCA (2015)^® software (Version 0.40.0) [36], which primarily relies on its default database in conjunction with well-known databases such as Eco Invent, GaBi, and the USLCI database. The assessment concentrated on building elements such as the substructure, superstructure, and enclosure (Figure 13) and covered both lifecycle stages A1–A3 and A1–A5 (as shown in Figure 14). The key materials evaluated included timber, concrete, metals, glass, and insulation, as specified in the project construction documents provided by the 340+ Dixwell design team. The functional unit chosen for the study was 1 m² of the building area.

Primary data for the analysis were obtained from the design team, which included architectural plans, the Autodesk Revit^® model, and the Procore® submittal documents.

2. Methodology

The methodology applied in this study aligns with the principles set forth by the International Organization for Standardization (ISO) 21931.

Access to One Click LCA (2015)^® was obtained through a complimentary student license provided specifically for this research and was used according to the guidelines of One Click LCA (2015)^® [36]. The quantities of materials in the Revit^® model were converted into volumes (cubic feet) and surface areas (square feet) using Revit^®’s material takeoff feature. This process separated materials such as concrete, CLT panels, insulation, metals, and glass for elements like walls, foundations, floors, and roofs. The door and window sizes and counts are listed in the Family and Type section of the schedule in Revit^®. The material quantities were then exported to an Excel sheet for sorting. After sorting, the final quantities were manually input into the One Click LCA (2015)^® software. One Click LCA (2015)^® [36] requires specific material types, which were identified in the procurement submittals. Once the materials and their Environmental Product Declarations (EPDs) were located, they were selected in One Click LCA (2015)^® [36], and the material quantities were entered manually.

The findings included the embodied carbon emissions and biogenic carbon associated with five key construction materials—timber, concrete, metals, glass, and insulation—utilized in the building’s enclosure, superstructure, and substructure. The findings from the 340+ Dixwell study were compared with other multi-family mass timber buildings, such as Adohi Hall and Heartwood, to gain a deeper understanding of the results.

These embodied carbon emissions results were also compared with data from the CLF Embodied Carbon Emissions Benchmark Study. However, it is important to note that the data in this benchmark study were constrained by the embodied carbon emissions databases accessible to the research team and the LCA studies that could be reviewed and compiled within the timeframe of the research project (summer and fall of 2016). These were derived from non-uniform LCA studies that utilized varying building scopes, LCA datasets, and methodologies [37]. According to the ECHO report [38], life cycle stages, reference study periods, the scope of elements, normalization units, and building area definitions must be standardized for valid comparisons. However, other differences, such as data sources, LCA software, and uncertainty analysis, do not necessarily need to match exactly but should be carefully documented and interpreted. The CLF Embodied Carbon Emissions Benchmark study does not fully align with the criteria set by the ECHO report, as it relies on non-uniform LCA studies with varying scopes, datasets, and methodologies, which may limit the comparability of results.

Additionally, a search was conducted for mass timber buildings in North America, particularly those with Post-and-Beam construction that had undergone LCA studies. The Timber Use Intensity (TUI) of these buildings was identified through various research papers. If the TUI was not available in the published literature, the researchers were directly contacted for the information. The TUI, along with the embodied carbon emissions and biogenic carbon associated with these mass timber buildings, was then compared to the findings of this study.

Several assumptions were made during the execution of this study and are outlined as follows:

Building Service Life: The building’s assumed service life was set at 60 years, which is in line with assumptions made in previous LCA studies. Materials such as concrete, insulation, and CLT panels do not require replacement; however, metal cladding and windows may need to be replaced within this timeframe.

Materials Selection: CLT (cross-laminated timber) panels make up a major part of the building. These panels were imported from Austria (Europe), and since their Environmental Product Declarations (EPDs) were not supported by the LCA tools, generic data from the European source (EU) were used. For the remaining construction materials, specific manufacturer EPDs were applied where available, and for those without EPDs, generic data specific to the United States were used.

3. Discussion

Table 1. Embodied carbon emissions and biogenic carbon storage (A1 to A5).

	Quantity (kg CO2 eq.)	Quantity (kg CO2 eq./m2)
Total embodied carbon (A1 to A5)	1,171,274	157
Total biogenic carbon storage	2,350,587	−316
Embodied carbon (A1 to A3)	982,939	132
Transportation to the building site (A4)	81,030
Construction/Installation process	107,304

shows that the total embodied carbon emissions for stages A1 to A5 amount to 1,171,000 kg CO₂ eq. (or 157.8 kg CO₂ eq./m²), while the total biogenic carbon storage is 2,350,000 kg CO₂ eq. (or −316.8 kg CO₂ eq./m²). Breaking it down, the embodied carbon emissions for the product stage (A1 to A3) are 982,000 kg CO₂ eq. (or 132.5 kg CO₂ eq./m²), for transportation to the building site (A4) is 81,000 kg CO₂ eq., and for the construction/installation process (A5) is 107,000 kg CO₂ eq.

Table 2. Total embodied carbon emissions (kg CO₂ eq) associated with the key construction materials.

Materials	Total Emissions (kg CO2 eq.)	Percentage (%)
Mass Timber	429,700	43.7
Concrete	236,000	24.0
Metals	138,800	14.1
Insulation	128,000	13.1
Glass	50,600	5.1

reveals the embodied carbon emissions associated with various materials (Figure 15). Mass timber leads with 4.29 × 10 ⁵ kg CO₂ eq. emissions, accounting for 43.7% of the total. Concrete is the next largest contributor, responsible for 2.36 × 10 ⁵ kg CO₂ eq., which represents 24.0% of the total emissions. Metals contribute 1.38 × 10 ⁵ kg CO₂ eq. (or 14.1%) while insulation accounts for 1.28 × 10 ⁵ kg CO₂ eq., contributing 13.1%, and glass emits 5.06 × 10 ⁴ kg CO₂ eq., contributing up to 5.1% of the total emissions.

Additionally,

Table 3. Various building components and their contribution to embodied carbon emissions.

Building Components	Embodied Carbon Emissions (kg CO2 eq.)	Percentage (%)
Superstructure	610,808	62.1
Enclosure	298,731	30.4
Substructure	73,400	7.5

shows various building components and their contribution to embodied carbon emissions. The results indicate that the superstructure is the largest contributor to embodied carbon emissions, accounting for approximately 62.1% of the total (Figure 16) due to the honeycomb CLT mass timber structural system. The enclosure follows, contributing 30.4% of emissions, with materials such as insulation, metal cladding, glass, concrete, and roofing. The substructure, which includes concrete footings, frost walls, and a slab-on-grade foundation, comprises concrete and has the smallest impact at 7.5%. These findings emphasize the significant role of the superstructure and enclosure in the building’s overall carbon footprint.

Figure 17 illustrates the carbon impact of 340+ Dixwell in terms of kg CO₂ eq. for stages A1–A3. Due to uncertainty regarding long-term carbon storage and end-of-life disposition, this study separately reports embodied carbon emissions and biogenic carbon sequestration. The left bar represents the embodied carbon emissions associated with different material categories: mass timber, concrete, metals, glass, and insulation. Each material’s contribution is stacked, with mass timber at the base, followed by concrete, metals, glass, and insulation, reaching a total of 9.82 × 10⁵ kg CO₂ eq. On the right side, biogenic carbon storage is depicted, showing a substantial negative carbon value (−2.35 × 10⁶ kg CO₂ eq. bio) that offsets the total embodied carbon emissions. While each material category—mass timber, concrete, metals, glass, and insulation—contributes to embodied carbon emissions, the biogenic carbon storage offsets these emissions. The substantial net negative carbon impact of the 340+ Dixwell project results from its high structural wood or mass timber content per square foot (TUI), owing to its honeycomb CLT design.

Comparison with Other Mass Timber Buildings (Stages A1 to A3: Product Stage):

Figure 18 shows the comparison between the GWP and biogenic carbon storage of the 340+ Dixwell project, Adohi Hall, and Heartwood. The graph demonstrates that Adohi Hall’s contribution to carbon emissions during the product stage (A1–A3) is 152.0 kg CO₂ eq./m² of the floor area [28], while the 340+ Dixwell project exhibited lower emissions at 132.5 kg CO2 eq./m². The University of Washington’s life cycle assessment (LCA) for Heartwood revealed that the global warming potential (GWP) for the A1–A3 stages was 159 kg CO₂ eq./m² [27]. This GWP is higher than that of both 340+ Dixwell (132.5 kg CO₂ eq./m²) and Adohi Hall (152 kg CO₂ eq./m²), with 340+ Dixwell having the lowest GWP among them.

The total carbon sequestered in the timber used for Heartwood is estimated to be −173 kg CO₂ eq./m². Likewise, the biogenic carbon storage for Adohi Hall is −186.1 kg CO₂ eq. per m², while 340+ Dixwell demonstrates significantly higher biogenic carbon storage at −316.8 kg CO₂ eq./m².

Table 4. The TUI, GWP, and biogenic carbon storage of various mass timber buildings.

Project	Type of Construction	Timber Use Intensity	Embodied Carbon (A1 to A3) (kg CO2 eq./m2)	Biogenic Carbon Storage (kg CO2 eq./m2)
340+ Dixwell, Connecticut	Honeycomb	1.22	132	−317
Heartwood Apartments, Seattle, WA	Post and Beam	0.70	159	−182
Adohi Hall, Fayetteville, Arkansas	Post and Beam	0.75	152	−186
Denver Office Building, Denver, Colorado	Post and Beam	0.92	121	−256
Return to Form MT	Post and Beam	0.75 (per 1 SF of above-podium area)	209	−209
Nez Perce USFS	Light Frame Hybrid	0.59	121	−165
Burwell-MT	Post and Beam	0.69	120	−192

presents the TUI for various mass timber buildings, along with their corresponding global warming potential and biogenic carbon storage. It highlights that the amount of carbon stored is directly proportional to the Timber Use Intensity (TUI). 340+ Dixwell has a TUI of 1.22 cubic feet of mass timber per square foot of the building area, which is higher than other mass timber buildings. Furthermore, this building achieves the highest level of biogenic carbon storage, exceeding the average biogenic carbon storage of other mass timber buildings listed in Table 4 by 60%.

The CLF Embodied Carbon Benchmark Study [37] reported that the initial embodied carbon (LCA Stage A) associated with a building’s structure, foundation, and enclosure is generally below 1000 kg CO₂ eq./m². For low-rise residential buildings (fewer than seven stories), the initial embodied carbon (LCA Stage A) of the structure, foundation, and enclosure is typically below 500 kg CO₂ eq./m².

The embodied carbon of 340+ Dixwell, measured at 157.8 kg CO₂ eq./m², is notably low compared to the ranges presented in the CLF Embodied Carbon Benchmark Study graph (Figure 19) for multi-family residential buildings. The graph shows that the median embodied carbon for multi-family buildings with one to six stories is 267.0 kg CO₂ eq./m². In comparison, the embodied carbon level of 340+ Dixwell is significantly lower at 157.8 kg CO₂ eq./m², representing a 40% reduction. This value highlights 340+ Dixwell’s remarkable efficiency in reducing embodied carbon. This outcome highlights the project’s successful implementation of strategies to reduce environmental impact compared to similar building types in North America.

Limitations and Future Recommendations

This study provides an initial assessment of the embodied carbon performance of honeycomb CLT structures, offering important insights into their potential for enhanced carbon storage and reduced environmental impact. However, several limitations and sources of uncertainty should be acknowledged.

The comparative LCAs in the literature review were based on Post-and-Beam mass timber, compared to conventional concrete and steel construction. Recent efforts, such as low-carbon concrete and green steel, offer reduced environmental impact from these structural systems; however, no comparisons were found between these approaches and mass timber construction, and they were not included in the literature review.

This study focuses on life cycle stages A1 to A3 (product stage) and A1 to A5 (product and construction stages) to allow for broader comparability. However, impacts from building operations (B modules) and end-of-life scenarios (C modules) were excluded from the system boundary. While this approach aligns with the primary goal of assessing embodied carbon, it omits downstream processes that can significantly influence the overall environmental performance of the building.

Uncertainty arises from assumptions related to material sourcing, transportation, and the use of default values within the selected LCA tool. Material-specific data were based on Environmental Product Declarations (EPDs) available in the software’s database. In cases where specific EPDs were unavailable, generic EPDs were used. To evaluate the impact, the actual EPD data for mass timber and the primary construction material imported from Austria were compared with the European EPD data used in this study. The difference in embodied carbon between the two was less than 5%.

Comparisons with the CLF Benchmark Study and other projects like Adohi Hall and Heartwood are informative but limited by inconsistencies in scope, data quality, and methodologies across sources. These factors may affect the comparability of the results and highlight the need for more uniform benchmarking standards.

As the first known study to evaluate honeycomb CLT-bearing wall and floor systems, it establishes a foundation for future work. Subsequent research should aim to verify these findings using a broader range of buildings and tools to ensure more comprehensive and comparable assessments. Future research should include a Life Cycle Cost Analysis (LCCA) to better understand the economic benefits of mass timber, such as energy savings and possibly lower maintenance costs. This study did not perform an LCCA due to limited cost data and factors like Passive House design, affordable housing requirements, and market changes during the pandemic. Including these in future studies would give decision-makers a clearer view of mass timber’s overall value.

4. Conclusions

In terms of embodied carbon emissions, numerous studies have demonstrated that mass timber structural systems in buildings are a more sustainable alternative to traditional structural systems of concrete and steel. There is potential to reduce the environmental impacts even within mass timber construction.

The A1 to A3 results for this honeycomb mass timber building show significantly lower embodied carbon emissions and higher biogenic carbon storage per square foot compared to conventional mass timber buildings such as Adohi Hall and Heartwood. Extending the analysis to stages A1 to A5, embodied carbon is approximately 40% lower than the median for other multi-family residential buildings, based on the Carbon Leadership Forum (CLF) Benchmark Study. Additionally, the building’s biogenic carbon storage per square foot is 60% greater than that of typical mass timber constructions, which is attributed to its higher Timber Use Intensity (TUI).

The findings of this study have meaningful implications for both practice and policy in the context of sustainable building design. From a policy perspective, there is a growing recognition of buildings as long-term carbon storage assets, as reflected in initiatives like ARPA-E’s HESTIA program, which promotes innovative approaches to carbon-storing construction. This analysis demonstrates that honeycomb mass timber construction can offer nearly double the TUI compared to conventional post-and-beam systems, significantly enhancing the carbon sequestration capacity of buildings. This suggests that building codes, green certifications, and carbon accounting frameworks should consider structural typology and TUI as critical factors in evaluating a building’s environmental performance. In practice, this insight will allow architects, engineers, and developers to prioritize material-efficient mass timber designs that maximize stored carbon while delivering sustainable structural outcomes.